1. Field of the Invention
The present invention relates to a copper-clad laminate and a printed wiring board. More particularly, the present invention relates to a printed wiring board having a fine-pitch wiring (circuit) pattern and exhibiting a high etching factor, and further relates to a copper-clad laminate which can be suitably employed in the production of such a printed wiring board.
2. Prior Art
The copper-clad laminate for use in the production of a printed wiring board is mainly obtained by bonding a copper loll as a conductive layer to one side or each of both sides of a substrate made from a glass-reinforced epoxy resin material or the like. This copper foil forms a wiring pattern by etching, etc. The copper foil and the substrate are bonded together under pressure while heating with or without an adhesive Interposed therebetween.
As such a copper foil, an electrolytically produced copper foil is generally employed. The electrolytic copper foil is obtained by separating a copper foil deposited on a drum. One side (glossy surface) of the electrolyte copper foil which has been detached from the drum, on which electrodeposition started, is relatively smooth, while the other side (rough surface) or matte surface of the electrolytic copper foil, on which electrodeposition has ended, is generally rough. In the sectional texture of tile electrolytic copper foil, the texture on the glossy surface side, i.e., the texture in the initial stage of electrodeposition exhibits random orientation and small crystal grains, and with the progress of electrodeposition the crystal texture comes to have greater orientation in the direction of the electrodeposition, with the crystal grain size becoming greater. In the conventional copper-clad laminates, the electrolyte copper foil is bonded on its matte surface side to the substrate for the reasons of greater bonding strength and easier handling. In order to improve the peeling strength from the substrate copper particles of 0.2 to 3.5 .mu.m in diameter are applied to the matte surface of the copper foil.
Formation of a wiring pattern by etching the above-mentioned copper-clad laminate has inevitably been accompanied by the problem that the etching factor becomes small due to the effects of crystal orientation and crystal grain boundary.
The etching factor being small means that sharp edges and desirable etching of the conductor pattern cannot be obtained. An explanatory view of the etching factor is shown in FIG. 1.
When the etching factor is small, the top is narrow and the bottom is broad. Thus, the spacing (gap) between the conductor pattern is reduced, thereby causing a problem that migration is likely to occur. Thus, formation of fine patterns is difficult.
Decreasing the thickness of the copper foil may be mentioned as a measure for obtaining a fine pattern. In this case, the sectional area of the conductor becomes so small that the quantity of available electric current is small. Accordingly, for allowing ample current passage, it is necessary to increase the sectional area of the conductor by performing copper plating on the wiring pattern obtained by the etching. Also, when the copper layer is rendered thin, lead bending occurs in a film carrier having an inner lead, e.g., TAB.
Moreover, in the etching of the above-mentioned conventional copper-clad laminate for fine pattern formation, electrodeposited copper particles remain adherent to the substrate of the printed wiring board at the foot of the conductor pattern formed by etching and the remaining electrodeposited copper particles cause the insulating resistance between conductors to decrease, in extreme cases causing short-circuiting.
Various reports have been published describing the formation of copper electrodeposits on the copper foil. For example, the electrodeposition of a layer of copper particles onto each of both sides of a copper foil has been reported. However, this copper foil is one for use as an inner conductor layer into a multilayer printed wiring board, and the purpose of the electrodeposition is to provide a substitute for the black oxide treatment aiming at ensuring the adhesion of the inner copper foil to the substrate at the secondary bonding. A pattern printing is applied to the glossy surface of the electrolytic copper foil, and the etching is performed from the glossy surface side. In another example, Japanese Patent Application Laid-Open Gazette No. 29740/1993 describes an electrolytic copper foil having both sides thereof provided with gloss plating, with one of the sides of the copper foil having been subjected to surface treatment, such as toughening. In this example, also, the etching is performed from the glossy surface side.